1. Field of the Invention The present invention relates to the hydraulic power controls of injection molding machines and, more particularly, to hydraulic power controls by means of which the quantity and pressure of the hydraulic fluid which is fed to the machine by a variable output pump are adjustable in accordance with a predetermined program of movement speed and torque or pressure, respectively.
2. Description of the Prior Art
Modern injection molding machines require hydraulic power for a number of separately powered drive units: the die closing unit drive, the plastification screw drive, the injection unit approach drive, the injection drive, the parts ejector drive, and the core removal drive. Each of these drives has specific requirements of movement force in the form of torque or pressure and movement speed, both changing in various ways in the course of each operational cycle of the machine.
For injection molding machines which are to be adjustable for different production setups, with different injection molding dies and different plastic raw materials to be injection molded, the various drive units need to be separately adjustable in terms of force and speed, in order to optimize the productivity and the power consumption of the machine. In the past, these hydraulic adjustability requirements have been fulfilled with only limited success, using conventional hydraulic control components, such as plunger type displacement valves, for the control of pressures and flow speeds, including digital, incrementally settable valves. The more recently suggested solutions, which feature so-called cartridge valves, are similarly handicapped by the fact that these valves, like the conventional plunger valves, are essentially of the open-and-shut type. For machines intended for high-quality production, it was therefore still necessary to provide a complex and costly hydraulic controls circuit with an electronic feedback loop and with all the necessary measuring and monitoring components.
In another prior art development, the suggestion is made to replace the feedback-adjusted hydraulic controls with pre-programmable electromagnetically adjustable pressure valves and flow control valves (Article by H. Rindt in Kunststoff-Journal, Volume 9, Apr. 4, 1975, pages 8-12: "Spritzgiessmaschinen -- programmierbar durch Proportionalventile"). This approach suggests the use of a continuously adjustable pressure control valve of the jet impact type, featuring a proportional-force electromagnet as the program input element. A two-way plunger valve, controlled by an auxiliary plunger and a cooperating proportional-force electromagnet, is used to adjust the flow speeds in both directions of movement, in accordance with a predetermined program which is imposed on the electromagnet.
It has now been found, as a result of extensive tests conducted with injection molding machines equipped with programmable electromagnetically adjusted valves, that the mere use of such valves in the hydraulic controls of the machine is still subject to certain operational shortcomings, and that additional requirements must be met, if the consumption of energy and the consumption of cooling liquid by the injection molding machine is to be reduced to the optimal minimum.